Electrodeposition method and product

ABSTRACT

Black iron oxide-containing coatings when deposited on solid substrates and cured exhibit sufficient electrical conductivity to permit application of a topcoat by electrodeposition. The black iron oxide-containing coatings can be formed on electrically conductive, solid substrates by electrodeposition. The coatings exhibit outstanding properties.

nited States Patent 1151 3,674,671 Stromberg 1 July 4, 1972 541ELECTRODEPOSITION METHOD AND 3,362,899 1/1968 Gilchrist ..204/l81PRODUCT 3,408,278 10/1968 St00dley ..204/18l Primary Examiner-Howard S.Williams Attorney-John W. Behringer, Eugene L. Bernard, Martin .1.Brown, James N. Dresser, W. Brown Morton, Jr., John T. Roberts, MalcolmL. Sutherland and Morton, Bernard, Brown, Roberts & Sutherland [57]ABSTRACT Black iron oxide-containing coatings when deposited on solidsubstrates and cured exhibit sufficient electrical conductivity topermit application of a topcoat by electrodeposition. The black ironoxide-containing coatings can be formed on electrically conductive,solid substrates by electrodeposition. The coatings exhibit outstandingproperties.

9 Claims, No Drawings ELECTRODEPOSITION METHOD AND PRODUCT Thisinvention relates to the formation of coatings on solid substrates withthe use of compositions having as an essential component black ironoxide which imparts electrical conductivity to the coatings therebypermitting the successful use of articles bearing such coatings assubstrates for further deposition of overlying coatings byelectrophoresis or electrodeposition. Moreover, the coatings containingblack iron oxide exhibit outstanding properties in terms ofelectrophoretic characteristics, adherence to substrates and corrosionresistance.

There are available in the art many descriptions of methods for coatingelectrically conductive objects by electrophoresis or electrodeposition,as well as many compositions which are suitable for use in suchprocedures. In the usual electrodeposition system, the article to becoated is placed in an aqueous bath and the article itself is eitherelectrically conductive or bears an electrically conductive coating. Theaqueous bath contains a water-dispersible, film-forming or coatingcomponent which is often an organic resin bearing carboxyl groups. Inthe system the article to be coated forms the anode in a direct currentcircuit with the vessel holding the aqueous bath or another conductor inthe bath serving as the cathode. Direct current from an outside sourceis applied through the bath via the electrodes and the film-formingmaterial becomes coated on the anode with the thickness of the resultingfilm being dependent on various factors such as the composition of thefilm-forming material, the applied voltage and the system geometry. Inany event, as coating proceeds, the resistance of the article or anodeto current'flow increases until a relatively uniform coating coversessentially the entire article. Completion of the coating process isgenerally indicated when the current becomes substantially constant. Bythis procedure it is convenient and economical to obtain uniform coatingof all portions of the article even when it has an intricate surfaceconfiguration.

In many instances it is desirable to form multiple coatings on solidsubstrates with at least one of the topcoatings, and preferably aplurality or even all of the coatings, being applied byelectrodeposition. If a topcoating is to be formed by electrodepositionthere isnecessitated the use of an undercoating which is electricallyconductive in order that the topcoating can be applied thereover byelectrophoresis. Such undercoatings can be formed by providing solid,electrically conductive particles in the coating composition. Theparticles must, however, not prevent the coating from forming arelatively nonconductive film as the undercoating is applied byelectrophoresis; yet when the coating is cured, it must exhibitsufficient electrical conductivity to permit application of a topcoatingby the electrodeposition technique. Moreover, the solid particles mustbe economically priced and must not unduly adversely affect thedesirable properties of the coating. Also, it is advantageous if thecoating containing such solids be amenable to application to articles,especially electrically nonconductive articles, by other techniques suchas spraying, dipping, etc., whereby upon curing of the coating thearticle can be further coated by electrodeposition. The topcoats may ormay not be electrically conductive after curing.

The present invention is based on the finding that black iron oxide canbe used as the essential electrically conducu've solids in coatingmaterials and outstanding results obtained when the cured coatingscontaining the black iron oxide are to be topcoated byelectrodeposition. The coatings resulting from the use of black ironoxide-containing, film-forming materials are satisfactorily electricallyconductive after curing of the coatings to enable further coating byelectrophoresis whether the black iron oxide bearing film is depositedon an electrically conductive article, for instance, byelectrodeposition, or on an electrically nonconductive article throughthe use of other techniques such as spraying, dipping, etc. In anyevent, articles bearing the black iron oxide-containing coating haveexhibited outstanding properties in terms of adherence to substrates,electrical conductivity, surface properties and corrosion resistance.

Thus the essential electrically conductive particles employed in thefilm-forming materials of the present invention are black iron oxidewhich is Fe o and is often available commercially in admixture withother iron oxides. The black iron oxide is usually a major proportion ofthe mixture which is preferably at least about 65, or often at leastabout 70, weight F6203. The black iron oxide is finely divided and isfrequently composed primarily of particles having a size of about 0.1 to1.5 microns, preferably about 0.25 to 0.5 microns, to enhance the easeof dispersing the solids in the aqueous bath and provide smoother andmore durable and homogeneous films. 1n the black iron oxide-containingcoatings of this invention and thus in the aqueous baths employed indepositing such coatings, the film-forming component, for instance, thecarboxyl group containing organic resin, is generally at least about 0.5part by weight per part of black iron oxide and may be up to about 10 ormore parts by weight per part of black iron oxide. Preferably, thefilm-forming material is about 2 to 4 parts by weight per part of blackiron oxide in the coating.

The black iron oxide exhibits satisfactory travel times in the aqueousbath during electrodeposition which is another advantageouscharacteristic when the electrically conductive coatings are formed bythis technique. Aside from imparting outstanding electrically conductivecharacteristics to the coatings of this invention the use of black ironoxide provides coatings of good salt spray and corrosion resistance andgood adhesion to substrates, as well as the property of good adhesion oftopcoats to the black iron oxide coating. Also, in applying the blackiron oxide-containing coatings by electrodeposition good voltageflexibility is possible and relatively high voltages can be toleratedwithout film disruption. The formation of topcoatings on the black ironoxide-containing coatings can be at relatively low voltages which canimprove the appearance and hiding power of the topcoats.

In the electrodeposition of the black iron oxide-containing coatings andin the formation of topcoats thereover by electrodeposition, conditionsknown in the art are suitable. Thus, the voltage is above the thresholddeposition voltage of the film-forming material and generally does notexceed values just below the rupture voltage of the coating. Frequently,the voltages are of the order of about 20 or 500 volts, preferably about40 to 200 volts, and the voltage applied in forming topcoats is ofiengreater than that used in depositing the undercoat. Temperatures of theaqueous bath maintained during electrodeposition are generally about 60to 125 F. or more, preferably about 70 to 95 F. The film-formingmaterials and the black iron oxide and any other pigments or materials,which remain inthe cured film, are often a minor proportion, forinstance, about 2 to 35 weight percent, preferably about 5 to 15 weightpercent, based on the total weight of the aqueous bath. Thus, the watercan be about 65 to 98 weight percent of the aqueous bath, preferablyabout to weight percent. Of course, the bath contains the anode andcathode in the usual spaced apart relationship and the bath can beagitated to maintain uniform temperatures and dispersion of the coatingcomponents. After electrodeposition, the coating is cured, for instance,at temperatures up to about 500 F., preferably about 200 to 400 F., forsuitable periods of time, for instance, about 2 to 60 minutes.

The cured film-forming material bearing the black iron oxide formed as acoating on the anode, is sufficiently electrically conductive, whetherthe coating has been applied by electrodeposition or by other means, topermit application of an overlying film-forming material byelectrodeposition. Where the initial coating on the article is to beapplied by electrodeposition the article or anode must be electricallyconductive and is usually metallic in nature. Suitable nonconducu'vesubstrates can also be used providing an electrically conductive coatingis applied by a suitable technique, and often the nonconductivematerials may be composed of wood, plastic, glass or other materials.Film thicknesses formed by electrodeposition are frequently about 0.5 to1.5 mils, preferably about 0.7 to 1 mil.

The filmforming materials employed in the black iron oxide-containingand other coatings applied by this invention are water thinnable orwater-soluble or can be made so, and are those materials oflencharacterized as electrodepositable, film-forming materials. Thus, thefilm-forming materials form current-carrying anions in the bath anddeposit on the anode in a form in which they are relatively waterandelectrically resistant materials. The film-forming material acts asa'binder for the pigment of the coating and frequently the film-formingmaterial is an organic resin. Many of these film-forming materials areknown in the art and are oflen composed to a major extent of a syntheticresin made from one or more carboxylic acids or their anhydrides oresters. Among the useful resins are those having an electricalequivalent weight between about 1,000 and 20,000 and an acid number oracid value of about 20 to 300, preferably an acid number of about 30 to150. Electrical equivalent weight is described in US. Pat. No.3,230,162, herein incorporated by reference.

The film-forming resins which can be applied by the coating techniquesof this invention can be formed by polymerization of, for instance,ethylenically-unsaturated, carboxyl group containing monomers, withother ethylenically unsaturated monomers. Frequently, the lattermonomers have up to about 30, preferably up to about 12, carbon atoms,while the ethylenically unsaturated, carboxyl group-containing monomersof ten have up to about 20 carbon atoms but may contain considerablymore, for instance, in the case of polyesters formed from unsaturatedfatty acids and polyols, with or without drying and semi-drying oils.Although the carboxylic group-containing monomer can be monocarboxyl,e.g. acrylic acids and esters and lower alkylsubstituted acrylic acidsand esters, it is preferred that it be composed to a major extent ofpolycarboxyl materials, i.e. having two or more groups, often havingfour to about 12 carbon atoms such as trimellitic acid, adipic acid,maleic anhydride, itaconic acid, phthalic anhydride, itaconic anhydride,fumaric acid, etc. and their lower alkyl esters. The other ethylenicallyunsaturated monomers usually comprise a minor amount, say up to about 25or more weight percent, of the resin and include many materials,especially vinyl members such as styrene, acrylonitrile, butadiene,vinyl toluene, butylenes, octenes and similar monomers. The drying oilsand semi-drying oils are represented by, for instance, linseed oil,oiticica oil, safflower oil, perilla oil, tung oil, soybean oil andother suitable oils including bodied oils. The polyol component in theoils is generally glycerol since the oils for the most part areavailable as glycerides. Other useful polyols such as ethylene glycol,neopentyl glycol, pentaerythritol and trimethylopropane can be employedin modifying the oils or in forming polyesters, and these polyols oftenhave up to about 12 carbon atoms. The polyol component can also be apolyester polyol.

The various film-forming polymers employed in this invention can beextended with materials which form thermosetting coatings such asnon-heat-reactive phenol aldehyde resins, amine-aldehyde condensationproducts and polyepoxy components. Thus, among the various suitablefilm-forming resins, are the alkyd resins and their mixtures withamine-aldehyde or phenolic materials, with or without furthermodification with vinyl monomer. Varieties of these resin formingmaterials are described in British Pat. Nos. 1,096,067 and 1,081,767 andUS. Pat. Nos. 3,230,162; 3,362,899 and 3,364,162, all incorporatedherein by reference. These various resinous materials are usuallypresent in the aqueous coating baths of this invention as dissolvedsalts, preferably volatile nitrogenous base salts of the resin such asthe ammonium salts or water-soluble aliphatic amine salts whose saltcomponents are relatively volatile and can be removed during curing ofthe coatings.

As previously noted, the coating compositions, whether black ironoxide-containing or not, employed in the method of this invention cancontain other materials such as other solid pigments, paint-type driers,surfactants to aid in dispersing the film-forming and pigmentcomponents, non-ionic organic liquids to assist in solubilization of thefilm-fomring resin components and other desirable ingredients. lnpigmented coatings, whether black iron oxide containing or not, theamount of film-forming material to pigment weight ratio is often about0.25 to 20:1, preferably about 1 to 5: 1.

This invention will be illustrated by the following specific examples.

EXAMPLE l 6,528 grams of maleinized drying oil resin prepared fromlinseed oil modified with about 15 weight percent maleic anhydride, werecombined with 1,1 10 milliliters of (29 percent) ammonium hydroxide,1,450 milliliters of n-butoxy ethanol and 366 milliliters of deionizedwater, giving a vehicle at percent solids having a pH of about 7.5 to8.0 and an acid value of 100. This resin vehicle was employed to producea conductive black primer for use in electrodeposition by combining thefollowing:

545 parts by weight of the vehicle 170 parts by weight of finely dividediron oxide containing about 76 weight percent black iron oxide Fr-. 0and 24 weight FeO 2.4 parts by weight of 6 percent cobalt naphthenate2.4 parts by weight of 6 percent manganese naphthenate 19.5 parts byweight of 24 percent lead naphthenate 6,165 parts by weight of deionizedwater Electrically conductive films were deposited on steel panels (zincphosphate-treated Bonderite 37) using an aqueous bath containing theforegoing described black primer in an amount sufi'lcient to give 8weight percent total nonvolatiles in the bath. 1n the electrodepositionthe conditions employed were 50 volts for 60 seconds at F. Afterdeposition, the films were cured by baking for 30 minutes at 300 F. andthe resultant films were approximately 0.8 mil thick.

A second resin was prepared from 5,240 grams of trimellitic anhydride,7,090 grams of neopentyl glycol and 3,980 grams of adipic acid bycooking the mixture at 370 F. to an acid value of 55 to 60. Theresulting material was mixed with 1,500 milliliters ofdimethylethanolamine, 3,140 milliliters of ethyl ketone, 3,760 grams ofCymel XM-l 1 16 which is a percent non-volatile amino cross-linkingagent and sufficient deionized water to give a vehicle of 40 percentsolids and a pH of 7.0 to 7.5.

One hundred and fifty parts by weight of the resulting vehicle, 25 partsby weight of titanium dioxide and 860 parts of deionized water werecombined to give a mixture containing 8 percent total solids. Thiscomposition was then employed to form white enamel topcoats over theelectrically conductive black primer coat on the steel panels. Duringthe electrodeposition to provide the topcoat, the conditions were voltsfor 60 seconds at 75 F. The white topcoat films were cured for 30minutes at 300 F. to give coatings with greater than 80 gloss, very goodcorrosion resistance, and excellent hardness and flexibility.

EXAMPLE II A resin was prepared from 1,301 grams of tall oil fattyacids, 1,482 grams of phthalic anhydride, 647 grams of pentaerythritoland 357 grams of ethylene glycol. The mixture was cooked at 390 F. to anacid value of 40. The resulting resin was combined with 254 millilitersof dimethyl ethanolamine, 442 milliliters of butoxy diethylene glycol,790 grams of Resimene 740, a high solids alcohol-modifiedmelamine-formaldehyde resin, and sufi'rcient deionized water to give asolution of 75 percent solids and a Ph of 7.5 to 8.0. Two coatingcompositions were prepared from this vehicle. The first coating was aconductive black iron oxide-containing material and the second coatingwas a white enamel formulation. The coatings were formulated as follows:

350 parts by weight of the iron oxide mixture of Example I 12,650 partsby weight of deionized water; and 2 522 parts by weight of the abovevehicle 172 parts by weight of titanium dioxide 3,910 parts of deionizedwater Using composition 1 electrically conductive films were depositedon zinc phosphate-treated steel panels at 75 volts for 60 seconds at 75F. The resulting films were cured at 300 F. for 30 minutes and hadthicknesses of about 0.7 to 0.9 mil. The white enamel topcoat,composition 2, was then deposited by electrodeposition over theelectrically conductive base coat at 100 volts for 60 seconds at 75 F.The enamel topcoat was cured at 300 F. for 30 minutes and exhibitedoutstanding properties with respect to coverage and corrosionresistance.

EXAMPLE HI A steel panel coated with an electrically conductive primeraccording to Example 11 was subsequently covered by a white topcoathaving the composition of that described in Example 1. The white filmwhich was applied at 125 volts for 60 seconds at 75 F. and cured at 300F for 30 minutes had very good gloss and appearance.

EXAMPLE IV The initial vehicle of Example 1 having 70 percent solids anda pH of 7.5 to 8.0 was used to prepare a black iron oxide base coatingby combination of the following: i 907 parts by weight of the vehicle170 parts by weight of the iron oxide mixture of Example I 5,820 partsby weight of deionized water Employing the resulting film-formingmaterial, electrically conductive films were electrodeposited on steelpanels at 75 volts for 60 seconds at 75 F The films were cured at 300 F.for 30 minutes and subsequently coated with a white enamel formulationas illustrated in Example I. The white topcoat was deposited at 150volts for 60 seconds at 75 F. and subsequently cured at 300 F. for 30minutes, giving a film having good gloss and appearance characteristics.

EXAMPLE V This example illustrates the unsuitability of red iron oxideas a pigment for forming electrically conductive coatings byelectrodeposition. A resin was formulated by cooking at 390 F. to anacid value of 40, a mixture of 1,301 grams of tall oil fatty acids,1,482 grams phthalic anhydride, 647 grams of pentaerythritol and 357grams of ethylene glycol. The cooked resin was mixed with 254milliliters of dimethylethanolamine, 442 milliliters of butoxydiethylene glycol, 790 grams of Resimene 740 and sufficient deionizedwater to give a solution of 75 weight percent solids and a pH of 7.5 to8.0.

Employing this vehicle, two black and one red coating compositions wereformulated as follows:

1,000 parts by weight of vehicle 350 parts by weight of iron oxidemixture of Example 1 12,650 parts by weight of deionized water;

1,000 parts by weight of vehicle 128 parts by weight of carbon black9.847 parts by weight of deionized water; and

1,000 parts by weight of vehicle 368 parts by weight of red iron oxide12,607 parts by weight of deionized water. Films based on each ofcompositions 1, 2 and 3 were deposited on zinc phosphate-treated steelpanels by electrodeposition at 75 volts for 1 minute at 75 F. Theresulting films were cured at 300 F. for 30 minutes. The cured blackfilms from compositions l and 2 were electrically conductive,

and, for instance, a white enamel topcoat could be formed over theseelectrically conductive films as described in the foregoing examples. Onthe other hand, the cured red films from composition 3 were notelectrically conductive and thus an overlying topcoat could not beapplied by electrodepositron.

By using composition 1, black iron oxide-containing films were depositedon steel panels at 50, 75, and volts, respectively, for 60 seconds at 75F. The resulting films had thicknesses of 0.7 to 1.0 mil and smooth,homogeneous appearances. Composition 2 was deposited on steel panels atthe same voltages and other conditions and at 100 and 125 volts theformulation have films which were above 1 mil thickness and were nothomogeneous, although at 50 and 70 volts the films were smooth andhomogeneous. Thus, the black iron oxide-containing coatings exhibitedgreater voltage flexibility than those containing carbon black.Moreover, when the steel panels bearing the carbon black-containingfilms were covered with a white topcoat, higher voltages were necessaryto obtain acceptable appearance and hiding power than when coating thepanels with the black iron oxide-containing coating. The greatervoltages needed for the carbon black films also resulted in some casesin topcoats which were too thick and somewhat discolored.

Steel panels bearing the black iron oxide and carbon black films werealso subjected to evaluation with respect to salt fog resistance. Thefilm containing the black iron oxide exhibited better resistance sinceit remained in the salt fog chamber for approximately 200 hours beforefailure, while the film based on carbon black failed between 100 andhours. Also, the film containing the black iron oxide was especiallydesirable as a primer coating since the gloss it exhibited was not sogreat as to deleteriously afiect adhesion of a topcoat.

It is claimed:

1. A method for electrophoretically coating a solid substrate whichcomprises forming a cured, electrically conductive coating on saidsubstrate, said coating being comprised of a film-forming polymer andfinely divided black iron oxide pigment to impart electricalconductivity to said cured coating, and subsequently applying byelectrodeposition over said cured coating a coating ofelectrodepositable film-forming polymer.

2. The coated article of claim 1.

3. The method of claim 1 in which the electrically conductive coating isapplied to an electrically conductive substrate by electrodepositionfrom an aqueous bath of a carboxyl group containing, electrodepositablefilm-forming polymer.

4. The coated article of claim 3.

5. The method of claim 3 in which the subsequently applied coating is ofa carboxyl group containing polymer.

6. The coated article of claim 5.

7. The method of claim 5 in which the weight ratio of filmformingpolymer to black iron oxide is about 2 to 4:1.

8. The method of claim 7 in which the subsequently applied coating ispigmented.

9. The coated article of claim 8.

a m :r a m

2. The coated article of claim
 1. 3. The method of claim 1 in which theelectrically conductive coating is applied to an electrically conductivesubstrate by electrodeposition from an aqueous bath of a carboxyl groupcontaining, electrodepositable film-forming polymer.
 4. The coatedarticle of claim
 3. 5. The method of claim 3 in which the subsequentlyapplied coating is of a carboxyl group containing polymer.
 6. The coatedarticle of claim
 5. 7. The method of claim 5 in which the weight ratioof film-forming polymer to black iron oxide is about 2 to 4:1.
 8. Themethod of claim 7 in which the subsequently applied coating ispigmented.
 9. The coated article of claim 8.